1. Field of the Invention
The present invention relates to a developing apparatus which is used in an image forming apparatus such as an electrophotographic or electrostatic recording copying machine, printer, or the like, and develops an electrostatic latent image on an image carrier.
1. Related Background Art
An electrophotographic image forming apparatus such as a laser beam printer, copying machine, or the like, visualizes an electrostatic latent image formed on the surface of an image carrier by developing it using a particle-like developing agent, thus forming a visible image on the surface of the image carrier. The developing agent is stored in a developing container as a developing agent chamber, and is conveyed to and held on a developing agent carrier which carries the developing agent, via a developing agent convey means. The layer thickness of the developing agent held on the developing agent carrier is controlled by a developing agent layer control member, and a predetermined charge is applied to the developing agent layer. Then, the developing agent is conveyed to a developing region where the image carrier and developing agent carrier oppose each other. The developing agent on the developing agent carrier moves to an electrostatic latent image forming portion on the image carrier in the developing region to develop and visualize the latent image on the image carrier.
The developed visible image is transferred onto a transfer medium such as a paper sheet by a transfer means, and is fixed by a fixing device by heating, compression, and the like. With a series of image forming processes described above, the user can obtain a desired image.
In the above-mentioned image forming apparatus, after a certain amount of printing, the developing agent in the developing container is consumed and its residual amount becomes small. When the residual amount of developing agent becomes small or zero, the developing agent cannot be sufficiently supplied to the developing region, and an image has image forming errors such as local image omission, or the like, or cannot be printed. Hence, the image forming apparatus has a developing agent residual amount detector for warning a small residual amount of developing agent to the user before the apparatus reaches an unprintable state or the like. As a known developing agent residual amount detection method, for example, changes in the amount of developing agent in the developing container are detected as changes in capacitance, thus detecting the amount of developing agent.
The developing agent residual amount detection method for detecting the amount of developing agent by detecting changes in amount of developing agent as changes in capacitance will be explained below with reference to FIG. 10. A developing apparatus for developing an electrostatic latent image on an image carrier 30 has a developing container for storing a developing agent, and a developing sleeve 6 serving as a developing agent carrier which is disposed at a position opposing an image carrier of the developing container and carries the developing agent. An AC power supply 7 for applying a predetermined developing bias is connected to the developing sleeve 6, so that the AC power supply 7 can apply a developing bias obtained by superposing an AC voltage on a DC voltage to the developing sleeve 6.
A signal detection means 18 is disposed in the developing container at a position opposing the image carrier with a predetermined spacing. The signal detection means 18 extends parallel with the developing sleeve 6 and consists of a rod-like conductive member. The signal detection means 18 is connected to a developing agent residual amount detection circuit 140 for detecting the amount of developing agent based on a developing agent residual amount detection signal produced by a capacitance that changes in correspondence with the amount of developing agent present between the signal detection means 18, and the developing sleeve 6 applied with the developing bias. The signal detection means 18 and the developing agent residual amount detection circuit 140 build a developing agent residual amount detector for detecting the residual amount of developing agent in the developing container.
The developing agent residual amount detection circuit 140 comprises a rectifying circuit 8 for rectifying the current induced at the signal detection means 18 in correspondence with the amount of developing agent present between the signal detection means 18 and developing sleeve 6, a current-voltage conversion circuit 9 for converting a current signal generated by the rectifying circuit 8 into a voltage Vdc, a capacitor 21 connected in parallel with the developing sleeve 6 with respect to the AC power supply 7 and having a predetermined capacitance Cs, a rectifying circuit 10 connected in series with the capacitor 21, and a current-voltage conversion circuit 11 for converting a current signal generated by the rectifying circuit 10 into a voltage Vs. The circuit 140 also has a comparator 19 for comparing the output voltages Vdc and Vs from the current-voltage conversion circuits 9 and 11, and producing an output when these two voltages match each other, and a CPU 20 for receiving the detection signal from the comparator.
Using the developing agent residual amount detection circuit 140 with the above-described arrangement, the output voltage Vdc generated in correspondence with the amount of developing agent between the developing sleeve 6 and signal detection means 18 is compared with the reference voltage Vs corresponding to the capacitance Cs of the capacitor 21, and the CPU 20 performs processing, e.g., a display of a developing agent amount warning or the like using the detection signal from the comparator 19, which is generated when the two voltages Vdc and Vs match each other. By selecting a capacitor having a capacitance corresponding to the amount of developing agent to be detected, an arbitrary detection level can be set.
In response to a short developing agent amount warning, the user replenishes more developing agent to the developing container, or removes a process cartridge including the developing container from the image forming apparatus main body, and shakes and inserts the process cartridge again, so that a sufficient amount of developing agent required for development is supplied to the developing region, thereby preventing image forming errors such as local image omission or the like or an unprintable state.
The developing agent residual amount detector of such capacitive detection method is popularly used since it has a simple arrangement and can be manufactured with low cost. In order to improve the detection precision in this method, a broad dynamic range of the output voltage must be assured. For this purpose, the signal detection means is preferably disposed as close as possible to the developing sleeve.
However, when the signal detection means is too close to the developing means, it readily disturbs developing agent supply to the developing sleeve, and poses another problem, e.g., an image density drop or the like. When the signal detection means is farther from the developing sleeve, the above-mentioned image problem hardly occurs, but the dynamic range of the output voltage narrows down, thus lowering the detection precision. When the detection precision lowers, local image omission is produced before a short developing agent amount warning is produced to the user, or a short developing agent amount warning is produced even though the developing container stores a sufficient amount of developing agent. Hence, the positioning precision of the signal detection means must be managed very strictly, resulting in a small margin in design.